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<span id="Cycle-Counters-1"></span><h3 class="section">10.3 Cycle Counters</h3>
<span id="index-cycle-counter-1"></span>

<p>FFTW&rsquo;s planner actually executes and times different possible FFT
algorithms in order to pick the fastest plan for a given <em>n</em>.  In
order to do this in as short a time as possible, however, the timer must
have a very high resolution, and to accomplish this we employ the
hardware <em>cycle counters</em> that are available on most CPUs.
Currently, FFTW supports the cycle counters on x86, PowerPC/POWER, Alpha,
UltraSPARC (SPARC v9), IA64, PA-RISC, and MIPS processors.
</p>
<span id="index-compiler-3"></span>
<p>Access to the cycle counters, unfortunately, is a compiler and/or
operating-system dependent task, often requiring inline assembly
language, and it may be that your compiler is not supported.  If you are
<em>not</em> supported, FFTW will by default fall back on its estimator
(effectively using <code>FFTW_ESTIMATE</code> for all plans).
<span id="index-FFTW_005fESTIMATE-3"></span>
</p>
<p>You can add support by editing the file <code>kernel/cycle.h</code>; normally,
this will involve adapting one of the examples already present in order
to use the inline-assembler syntax for your C compiler, and will only
require a couple of lines of code.  Anyone adding support for a new
system to <code>cycle.h</code> is encouraged to email us at <a href="mailto:fftw@fftw.org">fftw@fftw.org</a>.
</p>
<p>If a cycle counter is not available on your system (e.g. some embedded
processor), and you don&rsquo;t want to use estimated plans, as a last resort
you can use the <code>--with-slow-timer</code> option to <code>configure</code> (on
Unix) or <code>#define WITH_SLOW_TIMER</code> in <code>config.h</code> (elsewhere).
This will use the much lower-resolution <code>gettimeofday</code> function, or even
<code>clock</code> if the former is unavailable, and planning will be
extremely slow.
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